The present invention relates generally to electrical cable connectors and relates particularly, but not exclusively, to separable connector modules for connecting together the operating components of an underground power distribution system by means of shielded electrical cable.
Separable connector assemblies for underground power distribution cable, or shielded cable, are watertight when assembled and may be readily separated into two or more units to break a cable connection. As such units are available separately commercially for various reasons and are individually subject to special design considerations, they are commonly referred to as "modules". Thus, a connection includes two or more matching modules assembled together.
One type of separable connector commonly used is a "rod and bore" switching type. A switch module having a receiving bore tube, or snuffer, situated within a passageway in a shielded, insulating housing and a grasping contact member at the interior end of the bore tube receives a matching rod connector module having a rod contact which is inserted in the bore and grasped by the bore contact member. Examples of this type of connector are described, for example, in the following U.S. Pat. Nos.:
3,513,437 ISSUED 19 May 1970 to W. A. Morris
3,542,986 ISSUED 24 Nov. 1970 to E. J. Kotski
3,551,587 ISSUED 29 Dec. 1970 to R. F. Propst, and
3,587,035 ISSUED 22 June 1971 to E. J. Kotski
It is desirable to be able to operate such connectors while their cables are energized to interrupt the power. As the cables are generally carrying power at a voltage on the order of thousands of volts, separation of the contacts of the connector on a live cable results in the formation of an electric arc between the contacts. The arc will, unless promptly extinguished, eventually strike a ground plane such as the grounded shielding of the modules, and create a direct line-to-ground fault.
In the present connectors, the bore of a snuffer tube is lined with ablative material, a material which generates arc-extinguishing gases when subjected to an electric arc. An arc follower, a rod-shaped extension at the end of the metal contact rod and generally somewhat smaller in diameter than the rod, is also of ablative material. When the contact rod is pulled from the contact member of the bore module, the resulting arcing passes between the follower and the snuffer liner. The exposure of the ablative material to arcing causes it to generate arc-extinguishing gases which rapidly extinguish the arc. This permits the connector to be utilized as a switch by being operated under live conditions, without creating a line-to-ground fault.
There are certain failure modes associated with connectors of the type described above, and these failure modes relate, among other things, to the type of circuit which the connector is called upon to interrupt.
When the connector is interrupting a normal distribution circuit, there may simply be a failure to extinguish to arc as the rod is pulled from the bore. This will eventually establish arcing to a nearby ground plane surface. There may also occur a momentary clearing of the current as the rod is pulled from the bore, with a later restriking of the arc to a ground plane as the rod is fully removed from the bore and hot, ionized gases rush out of the bore in a conducting and unconfined state and establish a conducting path from the contacts to the shielding or other ground plane.
When the connector is interrupting a highly capacitive circuit, on the other hand, there can occur a different type of failure mode involving a restrike of the contacts. In this mode there is a momentary clearing of the current when the contacts are first separated, with a restriking of the arc between the contacts shortly thereafter. Such clearing and restriking may occur repeatedly as the contacts are separated until the arc between the contacts eventually finds a nearby ground plane. This mode will be described in more detail later.